If you want to graph the results, and/or store them, you can avoid some wheel re-invention, and use tools specifically designed to gather periodic, increasing data, and graphing it. The basic tool is rrdtool.

Once you understand how that works, you may want to implement the a package that makes it easy to configure and display the info via http. Take a look at cacti for this purpose. It may appear a little intimidating at first look, but once implemented, it is beautiful (unfortunately, it is done in PHP, not perl, but it is still very good).